DESCRIPTION

[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the
requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to
the ISO C standard.

The fprintf() function shall place output on the named output stream. The printf() function shall place
output on the standard output stream stdout. The sprintf() function shall place output followed by the null byte,
'\0', in consecutive bytes starting at *s; it is the user's responsibility to ensure that enough space is
available.

The snprintf() function shall be equivalent to sprintf(), with the addition of the n argument which states
the size of the buffer referred to by s. If n is zero, nothing shall be written and s may be a null pointer.
Otherwise, output bytes beyond the n-1st shall be discarded instead of being written to the array, and a null byte is
written at the end of the bytes actually written into the array.

If copying takes place between objects that overlap as a result of a call to sprintf() or snprintf(), the results
are undefined.

Each of these functions converts, formats, and prints its arguments under control of the format. The format is a
character string, beginning and ending in its initial shift state, if any. The format is composed of zero or more
directives: ordinary characters, which are simply copied to the output stream, and conversion specifications, each of
which shall result in the fetching of zero or more arguments. The results are undefined if there are insufficient arguments for the
format. If the format is exhausted while arguments remain, the excess arguments shall be evaluated but are otherwise
ignored.

[XSI]
Conversions can be applied to the nth argument after the format in the argument list, rather than to the next unused
argument. In this case, the conversion specifier character % (see below) is replaced by the sequence
"%n$", where n is a decimal integer in the range [1,{NL_ARGMAX}], giving the position of the
argument in the argument list. This feature provides for the definition of format strings that select arguments in an order
appropriate to specific languages (see the EXAMPLES section).

The format can contain either numbered argument conversion specifications (that is, "%n$" and
"*m$"), or unnumbered argument conversion specifications (that is, % and * ), but not
both. The only exception to this is that %% can be mixed with the "%n$" form. The results of
mixing numbered and unnumbered argument specifications in a format string are undefined. When numbered argument
specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the
(N-1)th, are specified in the format string.

In format strings containing the "%n$" form of conversion specification, numbered arguments in the
argument list can be referenced from the format string as many times as required.

In format strings containing the % form of conversion specification, each conversion specification uses the first
unused argument in the argument list.

[CX] All
forms of the fprintf() functions allow for the insertion of a language-dependent radix character in the output string. The
radix character is defined in the program's locale (category LC_NUMERIC ). In the POSIX locale, or in a locale where the
radix character is not defined, the radix character shall default to a period ( '.' ).

Each conversion specification is introduced by the '%' character [XSI] or by the
character sequence "%n$", after which the
following appear in sequence:

Zero or more flags (in any order), which modify the meaning of the conversion specification.

An optional minimum field width. If the converted value has fewer bytes than the field width, it shall be padded with
spaces by default on the left; it shall be padded on the right if the left-adjustment flag ( '-' ), described below, is
given to the field width. The field width takes the form of an asterisk ( '*' ), described below, or a decimal
integer.

An optional precision that gives the minimum number of digits to appear for the d, i, o,
u, x, and X conversion specifiers; the number of digits to appear after the radix character for the
a, A, e, E, f, and F conversion specifiers; the maximum number of
significant digits for the g and G conversion specifiers; or the maximum number of bytes to be printed from a
string in the s[XSI] and S conversion
specifiers. The precision takes the form of a period ( '.' ) followed either by an asterisk ( '*' ), described
below, or an optional decimal digit string, where a null digit string is treated as zero. If a precision appears with any other
conversion specifier, the behavior is undefined.

An optional length modifier that specifies the size of the argument.

A conversion specifier character that indicates the type of conversion to be applied.

A field width, or precision, or both, may be indicated by an asterisk ( '*' ). In this case an argument of type
int supplies the field width or precision. Applications shall ensure that arguments specifying field width, or precision, or
both appear in that order before the argument, if any, to be converted. A negative field width is taken as a '-' flag
followed by a positive field width. A negative precision is taken as if the precision were omitted. [XSI] In format
strings containing the "%n$" form of a conversion specification, a field width or precision may be
indicated by the sequence "*m$", where m is a decimal integer in the range [1,{NL_ARGMAX}] giving
the position in the argument list (after the format argument) of an integer argument containing the field width or
precision, for example:

printf("%1$d:%2$.*3$d:%4$.*3$d\n", hour, min, precision, sec);

The flag characters and their meanings are:

'

[XSI]
The integer portion of the result of a decimal conversion ( %i, %d, %u, %f, %F,
%g, or %G ) shall be formatted with thousands' grouping characters. For other conversions the behavior is
undefined. The non-monetary grouping character is used.

-

The result of the conversion shall be left-justified within the field. The conversion is right-justified if this flag is not
specified.

+

The result of a signed conversion shall always begin with a sign ( '+' or '-' ). The conversion shall begin
with a sign only when a negative value is converted if this flag is not specified.

<space>

If the first character of a signed conversion is not a sign or if a signed conversion results in no characters, a <space>
shall be prefixed to the result. This means that if the <space> and '+' flags both appear, the <space> flag
shall be ignored.

#

Specifies that the value is to be converted to an alternative form. For o conversion, it increases the precision (if
necessary) to force the first digit of the result to be zero. For x or X conversion specifiers, a non-zero result
shall have 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g
, and G conversion specifiers, the result shall always contain a radix character, even if no digits follow the radix
character. Without this flag, a radix character appears in the result of these conversions only if a digit follows it. For
g and G conversion specifiers, trailing zeros shall not be removed from the result as they normally are.
For other conversion specifiers, the behavior is undefined.

0

For d, i, o, u, x, X, a, A, e,
E, f, F, g, and G conversion specifiers, leading zeros (following any indication
of sign or base) are used to pad to the field width; no space padding is performed. If the '0' and '-' flags both
appear, the '0' flag is ignored. For d, i, o, u, x, and X
conversion specifiers, if a precision is specified, the '0' flag is ignored. [XSI] If the
'0' and '" flags both appear, the grouping characters are inserted before zero padding. For other conversions,
the behavior is undefined.

The length modifiers and their meanings are:

hh

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer
promotions, but its value shall be converted to signed char or unsigned char before printing); or that a following
n conversion specifier applies to a pointer to a signed char argument.

h

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to a short or unsigned short argument (the argument will have been promoted according to the integer
promotions, but its value shall be converted to short or unsigned short before printing); or that a following
n conversion specifier applies to a pointer to a short argument.

l (ell)

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to a long or unsigned long argument; that a following n conversion specifier applies to a pointer to
a long argument; that a following c conversion specifier applies to a wint_t argument; that a following
s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a,
A, e, E, f, F, g, or G conversion specifier.

ll (ell-ell)

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to a long long or unsigned long long argument; or that a following n conversion specifier applies to
a pointer to a long long argument.

j

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a
pointer to an intmax_t argument.

z

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier
applies to a pointer to a signed integer type corresponding to a size_t argument.

t

Specifies that a following d, i, o, u, x, or X conversion specifier
applies to a ptrdiff_t or the corresponding unsigned type argument; or that a following n conversion
specifier applies to a pointer to a ptrdiff_t argument.

L

Specifies that a following a, A, e, E, f, F, g, or
G conversion specifier applies to a long double argument.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The conversion specifiers and their meanings are:

d, i

The int argument shall be converted to a signed decimal in the style "[-]dddd". The precision
specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it shall be
expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero shall be
no characters.

o

The unsigned argument shall be converted to unsigned octal format in the style "dddd". The
precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it
shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero
shall be no characters.

u

The unsigned argument shall be converted to unsigned decimal format in the style "dddd". The
precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it
shall be expanded with leading zeros. The default precision is 1. The result of converting zero with an explicit precision of zero
shall be no characters.

x

The unsigned argument shall be converted to unsigned hexadecimal format in the style "dddd";
the letters "abcdef" are used. The precision specifies the minimum number of digits to appear; if the value being
converted can be represented in fewer digits, it shall be expanded with leading zeros. The default precision is 1. The result of
converting zero with an explicit precision of zero shall be no characters.

X

Equivalent to the x conversion specifier, except that letters "ABCDEF" are used instead of "abcdef"
.

f, F

The double argument shall be converted to decimal notation in the style
"[-]ddd.ddd", where the number of digits after the radix character is equal to the
precision specification. If the precision is missing, it shall be taken as 6; if the precision is explicitly zero and no
'#' flag is present, no radix character shall appear. If a radix character appears, at least one digit appears before it.
The low-order digit shall be rounded in an implementation-defined manner.

A double argument representing an infinity shall be converted in one of the styles "[-]inf" or
"[-]infinity" ; which style is implementation-defined. A double argument representing a NaN shall be converted in
one of the styles "[-]nan(n-char-sequence)" or "[-]nan" ; which style, and the meaning of any
n-char-sequence, is implementation-defined. The F conversion specifier produces "INF",
"INFINITY", or "NAN" instead of "inf", "infinity", or "nan", respectively.

e, E

The double argument shall be converted in the style
"[-]d.ddde±dd", where there is one digit before the radix character
(which is non-zero if the argument is non-zero) and the number of digits after it is equal to the precision; if the precision is
missing, it shall be taken as 6; if the precision is zero and no '#' flag is present, no radix character shall appear. The
low-order digit shall be rounded in an implementation-defined manner. The E conversion specifier shall produce a number
with 'E' instead of 'e' introducing the exponent. The exponent shall always contain at least two digits. If the
value is zero, the exponent shall be zero.

A double argument representing an infinity or NaN shall be converted in the style of an f or F
conversion specifier.

g, G

The double argument shall be converted in the style f or e (or in the style F or E
in the case of a G conversion specifier), with the precision specifying the number of significant digits. If an explicit
precision is zero, it shall be taken as 1. The style used depends on the value converted; style e (or E ) shall
be used only if the exponent resulting from such a conversion is less than -4 or greater than or equal to the precision. Trailing
zeros shall be removed from the fractional portion of the result; a radix character shall appear only if it is followed by a digit
or a '#' flag is present.

A double argument representing an infinity or NaN shall be converted in the style of an f or F
conversion specifier.

a, A

A double argument representing a floating-point number shall be converted in the style
"[-]0xh.hhhhp±d", where there is one hexadecimal digit (which shall
be non-zero if the argument is a normalized floating-point number and is otherwise unspecified) before the decimal-point character
and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2,
then the precision shall be sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a
power of 2, then the precision shall be sufficient to distinguish values of type double, except that trailing zeros may be
omitted; if the precision is zero and the '#' flag is not specified, no decimal-point character shall appear. The letters
"abcdef" shall be used for a conversion and the letters "ABCDEF" for A conversion. The
A conversion specifier produces a number with 'X' and 'P' instead of 'x' and 'p'. The
exponent shall always contain at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2.
If the value is zero, the exponent shall be zero.

A double argument representing an infinity or NaN shall be converted in the style of an f or F
conversion specifier.

c

The int argument shall be converted to an unsigned char, and the resulting byte shall be written.

If an l (ell) qualifier is present, the wint_t argument shall be converted as if by an ls conversion
specification with no precision and an argument that points to a two-element array of type wchar_t, the first element of
which contains the wint_t argument to the ls conversion specification and the second element contains a null wide
character.

s

The argument shall be a pointer to an array of char. Bytes from the array shall be written up to (but not including) any
terminating null byte. If the precision is specified, no more than that many bytes shall be written. If the precision is not
specified or is greater than the size of the array, the application shall ensure that the array contains a null byte.

If an l (ell) qualifier is present, the argument shall be a pointer to an array of type wchar_t. Wide characters
from the array shall be converted to characters (each as if by a call to the wcrtomb() function, with the conversion state described by an mbstate_t object
initialized to zero before the first wide character is converted) up to and including a terminating null wide character. The
resulting characters shall be written up to (but not including) the terminating null character (byte). If no precision is
specified, the application shall ensure that the array contains a null wide character. If a precision is specified, no more than
that many characters (bytes) shall be written (including shift sequences, if any), and the array shall contain a null wide
character if, to equal the character sequence length given by the precision, the function would need to access a wide character one
past the end of the array. In no case shall a partial character be written.

p

The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printable characters, in
an implementation-defined manner.

n

The argument shall be a pointer to an integer into which is written the number of bytes written to the output so far by this
call to one of the fprintf() functions. No argument is converted.

Print a '%' character; no argument is converted. The complete conversion specification shall be %%.

If a conversion specification does not match one of the above forms, the behavior is undefined. If any argument is not the
correct type for the corresponding conversion specification, the behavior is undefined.

In no case shall a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the
field width, the field shall be expanded to contain the conversion result. Characters generated by fprintf() and
printf() are printed as if fputc() had been called.

For the a and A conversion specifiers, if FLT_RADIX is a power of 2, the value shall be correctly rounded to a
hexadecimal floating number with the given precision.

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the
given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with
the extra stipulation that the error should have a correct sign for the current rounding direction.

For the e, E, f, F, g, and G conversion specifiers, if the number of
significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded. If the number of significant
decimal digits is more than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits, then the result
should be an exact representation with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings
L < U, both having DECIMAL_DIG significant digits; the value of the resultant decimal string D should
satisfy L <= D <= U, with the extra stipulation that the error should have a correct sign for the
current rounding direction.

[CX] The
st_ctime and st_mtime fields of the file shall be marked for update between the call to a successful execution of
fprintf() or printf() and the next successful completion of a call to fflush() or fclose() on the same stream or a
call to exit() or abort().

RETURN VALUE

Upon successful completion, the fprintf() and printf() functions shall return the number of bytes transmitted.

Upon successful completion, the sprintf() function shall return the number of bytes written to s, excluding the
terminating null byte.

Upon successful completion, the snprintf() function shall return the number of bytes that would be written to s
had n been sufficiently large excluding the terminating null byte.

If an output error was encountered, these functions shall return a negative value.

If the value of n is zero on a call to snprintf(), nothing shall be written, the number of bytes that would have
been written had n been sufficiently large excluding the terminating null shall be returned, and s may be a null
pointer.

ERRORS

For the conditions under which fprintf() and printf() fail and may fail, refer to fputc() or fputwc().

In addition, all forms of fprintf() may fail if:

[EILSEQ]

[XSI]
A wide-character code that does not correspond to a valid character has been detected.

[XSI]
The value of n is greater than {INT_MAX} or the number of bytes needed to hold the output excluding the terminating null is
greater than {INT_MAX}.

The following sections are informative.

EXAMPLES

Printing Language-Independent Date and Time

The following statement can be used to print date and time using a language-independent format:

printf(format, weekday, month, day, hour, min);

For American usage, format could be a pointer to the following string:

"%s, %s %d, %d:%.2d\n"

This example would produce the following message:

Sunday, July 3, 10:02

For German usage, format could be a pointer to the following string:

"%1$s, %3$d. %2$s, %4$d:%5$.2d\n"

This definition of format would produce the following message:

Sonntag, 3. Juli, 10:02

Printing File Information

The following example prints information about the type, permissions, and number of links of a specific file in a directory.

The first two calls to printf() use data decoded from a previous stat() call.
The user-defined strperm() function shall return a string similar to the one at the beginning of the output for the
following command:

ls -l

The next call to printf() outputs the owner's name if it is found using getpwuid(); the getpwuid() function shall
return a passwd structure from which the name of the user is extracted. If the user name is not found, the program instead
prints out the numeric value of the user ID.

The next call prints out the group name if it is found using getgrgid(); getgrgid() is very similar to getpwuid() except that it shall return group information based on the group number. Once
again, if the group is not found, the program prints the numeric value of the group for the entry.

Printing a Localized Date String

The following example gets a localized date string. The nl_langinfo()
function shall return the localized date string, which specifies the order and layout of the date. The strftime() function takes this information and, using the tm structure for values,
places the date and time information into datestring. The printf() function then outputs datestring and the
name of the entry.

Printing Error Information

The following example uses fprintf() to write error information to standard error.

In the first group of calls, the program tries to open the password lock file named LOCKFILE. If the file already exists,
this is an error, as indicated by the O_EXCL flag on the open() function. If the call
fails, the program assumes that someone else is updating the password file, and the program exits.

The next group of calls saves a new password file as the current password file by creating a link between LOCKFILE and
the new password file PASSWDFILE.

Formatting a Decimal String

The following example prints a key and data pair on stdout. Note use of the '*' (asterisk) in the format string;
this ensures the correct number of decimal places for the element based on the number of elements requested.

Reporting an Event

The following example loops until an event has timed out. The pause() function
waits forever unless it receives a signal. The fprintf() statement should never occur due to the possible return values of
pause().

Printing Monetary Information

The following example uses strfmon() to convert a number and store it as a
formatted monetary string named convbuf. If the first number is printed, the program prints the format and the description;
otherwise, it just prints the number.

In the last line of the example, after processing three characters, nine bytes have been output. The fourth character must then
be examined to determine whether it converts to one byte or more. If it converts to more than one byte, the output is only nine
bytes. Since there is no fourth character in the array, the behavior is undefined.

APPLICATION USAGE

If the application calling fprintf() has any objects of type wint_t or wchar_t, it must also include the <wchar.h> header to have these objects defined.

CHANGE HISTORY

Issue 5

Aligned with ISO/IEC 9899:1990/Amendment 1:1995 (E). Specifically, the l (ell) qualifier can now be used with
c and s conversion specifiers.

The snprintf() function is new in Issue 5.

Issue 6

Extensions beyond the ISO C standard are marked.

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

The prototypes for fprintf(), printf(), snprintf(), and sprintf() are updated, and the XSI shading
is removed from snprintf().

The description of snprintf() is aligned with the ISO C standard. Note that this supersedes the snprintf()
description in The Open Group Base Resolution bwg98-006, which changed the behavior from Issue 5.

The DESCRIPTION is updated.

The DESCRIPTION is updated to use the terms "conversion specifier" and "conversion specification" consistently.

ISO/IEC 9899:1999 standard, Technical Corrigendum 1 is incorporated.

An example of printing wide characters is added.

End of informative text.

UNIX ® is a registered Trademark of The Open Group.
POSIX ® is a registered Trademark of The IEEE.
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